ML031050313

Memo from Mark Kirk to N Chokshi Re Status of ORNL Calculations of Burst Pressure for Cavities That Larger than Cavity Found at Davis-Besse

ML031050313
Person / Time
Site:	Davis Besse
Issue date:	08/05/2002
From:	Matthew Kirk Office of Nuclear Regulatory Research
To:	Nilesh Chokshi, Long S Office of Nuclear Reactor Regulation, Office of Nuclear Regulatory Research
References
FOIA/PA-2003-0018
Download: ML031050313 (5)

Text

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,~ - Lon -50saPage 1 August 5, 2002 MEMORANDUM From: Mark Kirk To:

Nllesh Chokshl !L Steve Long H/e Re:

Status of ORNL Calculations oF Burst Pressures for Cavities that are Larger than the Cavity Found at Davis Besse.

This menorandurn provdes the status of our analysis. Paul Williams (ORNL) Is hI the pros Cf preparing a letter report tat will containing all of the details; his report will be available next week.

1. The purpose of our analysis has been to assess how much larger the vty in the Davis Besse head could have been before rupture of the cladding was likely at pressures at or near the operating pressure. Fqure 1 shows the finite element model we have used in these calculations: it features a global model of the head and a ical model of the area near the wastage cavity. This global/local approach allowed us to better refine the mesh I way of the cavity. In all of our analyses we have used

a. A uniform cladding thidcness of 0.24-hi. (this was the minimum thickness reported by FENOC based on Ur measurements made on a V-in. square grid).

b. Tensile properties at 600°F that represent a lower bound to all avaIlable 308 SS results available at this temperature.

2. Figure 2 depicts the two patterns of wastage growth we have investigated. Our initial analyses used the "self similr" growth pattern. In performing these analyses we noted that the boundaries of the wastage cavity had expanded to interact with the boundaries or the bcal model. This caused us concern that the predicted burst pressures could be Influenced by the modeling approach we had adopted. To help assess this we performed some calculations with a more idealized growth model (the ellipsoidal model) wherein we could better control the growth of the cavity, keeping t away from the boundaries oF the local model.

3. Failure pressures predicted using both growth models (see DRAFT ORNL Letter Report dated 7 02, 'Stochastic Failure Model for the Davls-Besse RPN Head,' by Williams and Bass for mode details on failure pressure prediction) are compared hI Figure 3. FIgure 3 also shows the relationship between failure pressure and area predicted for drcular disks by a dosed-form plaicity model developed hI 1970 by Chakrabarty and Alexander (CA) (the error bars and error bounds are based on how well finite element analysis analysis was able to predict fallure of burst disks reported by Riccardella, again see the DRAFT ORNL Letter report of 7-15-02 for further detalls). The cose agreement between the burst pressures predicted for the two cavity geometries shown hI Figure 2 and the dosed-form CA formula suggests that cavity geometry Is not a factor that exerts significant control on the burst pressure.

4. Since cavity geometry does not appear to be a major variable, a first order appreciation for how much larger the cavity would have to be to rupture at pressures near the operating pressure can be obtained by thinking of these cavities as Wrcles, as described hI the following table. To express these results hi terms of time they obviously need to be used together with an appropriate comosion rate, and expressed In the context of an uncertainty analysis. It Is my understanding Bll Cullen Is currently In the process of establishing a contract with ANL to define corrosion growth rates. It Is also relevant to point out that there Is currently a disagreement between ourselves and FENOC regarding thesize of the original caity (FENOC is using en initial lC r rd v: as !c+ed d in cc K:1

^ :re i1ic Frenerm, information'

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I even Long-2 status memo.dc Page aea of 20' in2). We are in the proces of resolving this discepancy.

cavity Area [In']

Radius of an Growth of radius equivalent drde [in) (from original cavity) needed to cause faflure rinl Original cavity 36 3.4 NtA Cavty wlth a 5% fallure 165 7.2 3.9 pressure at the set-point pressure (2500 PSI) cayvty wtn a 5%

probability of rupture at the operating pressure 215 I

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6. In Figure 4 we compare e pesres predicted by SIA (FENOCs con ctor) with the CA theoreticl model. At least In terms of trends there Is good agreement between SIA's analysis and the CA theory, and I think It Is safe to say that STA Is predictig lwer burst pressures than would our analysis under Identical conditions. Clea*zy we could go into greater detail regarding why the SIA analysis Is providing the answers that It Is, and we are prepared to do that, If so requested.

7. It Is, of course, always possible to do more analysis (if warranted). Some features of these analyses that could be Improved (that I,, made more realistic) Include the following:

a. The geometry of the original cavity could be made to agree with the dimensions taken from the dental kIpressko made of the wastage cavity, once these dimensions become avallable.

b. The dimensions and sura;ce relW of the cladding could be made to agree with the dimensions taken from the dental Impression made of the wastage cavity, once these dimensions become available. This Improved cladding model could Indude modeling of the Inaeased cladding thklness due to the 3-groove weld (a feature which has currently been omitted)

c. The stess-straln properties of the cladding could be made to agree with measurements made on the Davis Besse cladding, once these measurements become available.

d. Nozzle 11, which becomes fully engulfed by the cavity by some of the large wastage areas, could be restraIned against motion in the vertical direction (in our current model the nozzle 'floats' freely on the cladding) to better model the restraints Imposed by structures above the RPV head. In the ORNL letter report of next week such an analysis will be reported. However we do not expect this change of boundary conditions to slgnlfcantly change the outcome of this analysis, largely because the wastage area Is already so lrge when nozzle 11 Is corroded away from the RPV head.

Please let me know If you have any questions or concrns

I Steven Long 502 status memo.doc Page:

Figure 1. Finite element model used by ORNL

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Comparison of SIA prediced burst pressures with theoretical predictions for drocular disks.